1. Field of the Invention
The present invention generally relates to a pressure sensing apparatus and a pressure measurement method using the same and, more particularly, to an array type pressure sensing apparatus and a pressure measurement method using the array type pressure sensing apparatus.
2. Description of the Prior Art
With the development in technology, the pressure sensing apparatus has been widely used in the daily lives. For example, Tekscan has used gray-scale pressure sensing in home care identification and mat pressure sensing. Quantization of touch reception for biomimetic electronic skin has been provided. The resistive touch panel has been commercialized. In consumer electronics, multi-touch technology developed by Apple and Nintendo's Wii Fit have built milestones in pressure sensing. Therefore, large-area multi-touch, array type, 3-D pressure identification will be the mainstream of future home care sensing and consumer electronics.
The conventional pressure sensing apparatus for medical or analytical purpose has been disclosed in, for example, U.S. Pat. No. 5,905,209 and Taiwan Patent No. 351990. In Taiwan Patent No. 351990, a plurality of independent pressure sensing units are disposed at the bottom of a shoe. The voltage signal of each pressure sensing unit is acquired by complicated circuitry. For a single shoe, at least 24 pressure sensing units are disposed with complicated read-out circuitry. If large-area sensing is required, the number of pressure sensing units has to be increased, which leads to high hardware cost and difficulty in manufacturing sensing devices on a large area.
Moreover, in U.S. Pat. No. 5,010,772, the array type capacitive sensing device is manufactured in a mattress shape to sense the pressure from a human body on the mattress. However, the scanning and driving architecture is easily affected by the leakage current due to minimal capacitive charge storage to cause error sensing and error identification in scanning and driving of the array type capacitive sensing device. Moreover, an additional correction circuit is required to correct the pressure corresponding to capacitance variation because of low precision in electric signals corresponding to pressure by the charge amplification sensing architecture.
The conventional multi-touch pressure sensing apparatus is disclosed, for example, in U.S. Pat. No. 5,505,072. The dynamic response scanning circuit in the array type pressure sensing device uses test signals with reference to voltage feedback to realize sensitivity and spatial resolution sensing. Moreover, the test scanning signal is controlled to scan the active region of the array type pressure sensing device. When the piezoresistive sensors are restructured due to an applied pressure, the sensing circuit picks up electric signals from the respective piezoresistive sensors. These electric signals are generated from voltage variations due to resistance variations in the re-structured sensors. A pressure gradient distribution over the respective sensors can be obtained from the above. However, in this method, the driving method for the sensing circuit and its algorithm are too complicated, further leading to delay in calculation of pressure variations.
The conventional touch sensor is disclosed, for example, in U.S. Pat. No. 5,159,159. The 2-D positioning and pressure sensing circuit of the touch sensor uses an array type resistive sensor with crisscross x-axis and y-axis. In the array type resistive sensor, a resistance-variable piezoresistive material is inter-layered. By constant current driving and scanning or constant voltage driving and scanning, touch positioning and pressure gray scale identification can be realized. However, resistive multi-touch positioning and multi-touch pressure gray scale identification cannot be realized by the above constant current driving and scanning or constant voltage driving and scanning based on the x-axis and y-axis.
FIG. 1A is an explosion view of a conventional array type pressure sensing apparatus. Referring to FIG. 1A, the conventional array type pressure sensing apparatus 100 comprises a top substrate 110 and a bottom substrate 120 with a plurality of resistive units 130 therebetween. Such an array type resistive touch device 100 has a 3-wire structure. Moreover, three traversal x-axes 112 are disposed on the top substrate 110 and three longitudinal y-axes 122 are disposed on the bottom substrate 120 so that each of 9 piezoresistive units 130 is disposed on the cross between one x-axis 112 and y-axis 122.
Generally, these resistive units are implemented using a sheet resistive unit 130′ in the array type pressure sensing apparatus 100′ as shown in FIG. 1A′. To make it easily understood, the piezoresistive units 130 in FIG. 1A are used for exemplification.
When the resistive units 130 are not applied with any pressure, resistance balance is achieved on both the x-axes 112 and the y-axes 122. However, when resistive units 130 are applied with a pressure, resistance value variation due to the applied pressure causes resistance unbalance. Under resistance unbalance, zero potential scanning is performed on the x-axes 112 or the y-axes 122 to obtain a voltage matrix on the x direction and a voltage matrix on the y direction. Then, coordinate encoding and coordinate operation are performed on the voltage matrix on the x-axes and the voltage matrix on the y-axes to obtain the coordinate positions and resistance values of these resistive units 130.
In order to better understand the above description, please refer to FIG. 1B, which is a circuit diagram of the array type pressure sensing apparatus in FIG. 1A. Referring to FIG. 1A to FIG. 1B, the x-axes 112 are scanned. More particularly, the x-axes 112a, 112b, 112c are input with voltage signals V, 2 V, 3 V, respectively. When the x-axis 112a is input with the V, the voltages on the y-axes 122 can be measured. For example, on the y-axis 122a, with an equivalent circuit as shown in FIG. 1C, when the x-axis 112a is input with the voltage signal V, the voltage signal V11 on the y-axis 122a can be measured, wherein R11 indicates the resistance value of the resistive units 130a and Rp is the overall effective resistance value of the other 8 piezoresistive units.
Similarly, the voltage signal V12 and V13 can be measured on the y-axes 122b and 122c, respectively. Moreover, when the x-axis 112b is input with the voltage signal 2 V, the voltage signals V21, V22, V23 can be measured on the y-axes 122a, 122b and 122c, respectively. When the x-axis 112c is input with the voltage signal 3 V, the voltage signals V31, V32 and V33 can be measured on the y-axes 122a, 122b and 122c, respectively. These voltage signals form an x-matrix, shown as follows:
                              [          X          ]                =                  [                                                                      V                  1                  1                                                                              V                  1                  2                                                                              V                  1                  3                                                                                                      V                  2                  1                                                                              V                  2                  2                                                                              V                  2                  3                                                                                                      V                  3                  1                                                                              V                  3                  2                                                                              V                  3                  3                                                              ]                                    (        1        )            
Similarly, the y-axes 122 are sequentially scanning to measure voltage signals on the x-axes to form a y-matrix. Then, coordinate encoding and coordinate operation can be performed on the x-matrix and the y-matrix to obtain the coordinate positions and resistance values of the resistive units 130.
However, such a method can only be used to sense the position where the pressure is applied on the sensor but fails to precisely provide the pressure values through a pressure distribution of the resistive touch panel. To further acquire the pressure distribution corresponding to precise pressure values, the resolution of the analog-to-digital converter in the micro-controller has to be improved, leading to complicated encoding operation and difficulty in realization of large-area, multi-linear scanning and driving nor high gray-scale pressure identification. Furthermore, the sensitivity of the resistive touch devices cannot be improved.
Therefore, there is need in providing an array type pressure sensing apparatus and a pressure measurement method using the array type pressure sensing apparatus to overcome the above problems.